Rattus norvegicusbrown rat(Also: Norway rat)

Geographic Range

Norway rats (Rattus norvegicus) are originally native to northern China. Following a series of introductions, the species had found its way to Eastern Europe by the early eighteenth century. By the year 1800, they occurred in every European country. Records show the first sighting of R. norvegicus in the New World occur in the 1770's as ship stowaways. Today, Norway rats (also known as brown rats) can be found on every continent of the world except Antarctica. (Nowak and Paradiso, 1983; Silver, 1927)


In Asia, R. norvegicus was native to forests and brushy areas. Today, however, Norway rats find preferred habitat to be alongside the rapid expansion of the human population. Nearly every port city in the world has a substantial population of these rodents. They occupy a variety of habitats including garbage dumps, sewers, open fields and woodlands, basements, and nearly anywhere else that food and shelter might be found. Anywhere that humans are located, R. norvegicus will most likely follow. (Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990)

Physical Description

Rattus norvegicus is a rather large member of the mouse family. On average, these rats reach nearly 400 mm nose-to-tail, and weigh 140 to 500 g. Males are usually larger than females. In natural populations, these rats are covered with coarse, brownish fur (sometimes splotched with black or white hairs) on their dorsal surface, which usually lightens to a gray or tan color nearing the underside. Various strains of these rats bred in captivity may be white, brown, or black. The ears and tail are bald. The length of the tail is shorter than the length of the body. Molars are lophodont and the dentary is 1/1-0/0-0/0-3/3. The ears of Norway rats are typically shorter than those of related species, and do not cover up the eyes when pulled down. Norway rats can be easily mistaken for black rats, however, the temporal ridges of the Norway rat are straight, whereas those of the black rat are curved. (Nowak and Paradiso, 1983; Avalos and Callahan, 2001; Calhoun, 1962; Nowak and Paradiso, 1983; Parker, 1990)

  • Sexual Dimorphism
  • male larger
  • Range mass
    140 to 500 g
    4.93 to 17.62 oz
  • Average mass
    400 g
    14.10 oz
  • Average length
    399 mm
    15.71 in
  • Average basal metabolic rate
    1.404 W


The mating system of R. norvegicus is best described as polygynandrous. Social animals, Norway rats tend to breed in large groups. Once a female enters her six-hour estrus period, she may mate as many as five-hundred times with competing males. (Parker, 1990)

Although not technically a seasonal breeder, a mating increase occurs in the warmer months of the year. An average female is capable of giving birth approximately seven times per year. Around 18 hours after giving birth, females experience postpartum estrus, and mate again. This reproductive function is responsible for the huge birthrates of Norway rats, which can reach 60 young each year per female. After a short gestation period of 22 to 24 days, the litter of approximately 8 pups is born. The young are very small and underdeveloped. It takes 14 to 17 days for the young's eyes to open. Newborns weigh an average of 5 grams and are milk-fed until weaning occurs at 3 to 4 weeks, and the young then leave the nest. (Barnett, 1963; Calhoun, 1962; Parker, 1990)

Often, the litters of numerous females will occupy the same nest, and all the young are cared for by the adults, regardless of who the true mothers are. This communal care makes the species something of a cooperative breeder. (Parker, 1990)

Males usually reach sexual maturity at 3 months and females at 4. However, it is usually the female who mates first because competition for mates among males prevents the smaller, less-dominant individuals from succeeding immediately. Rattus norvegicus is capable of mating for up to two years. (Calhoun, 1962; Parker, 1990)

  • Breeding interval
    Norway rats may breed up to 7 times per year
  • Breeding season
    Breeding occurs year round, but is less pronounced during colder months.
  • Range number of offspring
    2 to 14
  • Average number of offspring
  • Range gestation period
    22 to 24 days
  • Range weaning age
    3 to 4 weeks
  • Range time to independence
    4 to 5 weeks
  • Range age at sexual or reproductive maturity (female)
    3 to 4 months
  • Range age at sexual or reproductive maturity (male)
    3 to 4 months

Parental care is provided by females. Because these animals often nest communally, the litters of several different females often occupy the same nest. In nesting groups of more than one female, if a mother is killed, the other females will take over nursing the newborns. Males do not particupate in parental care. (Parker, 1990)

  • Parental Investment
  • no parental involvement
  • altricial
  • female parental care
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • protecting
      • female


The maximum lifespan of R. norvegicus is 4 years (in captivity). In the wild, it is assumed that they live for upwards of 2 years. (Nowak and Paradiso, 1983; Parker, 1990)

  • Range lifespan
    Status: captivity
    4 (high) years
  • Average lifespan
    Status: wild
    2 years
  • Typical lifespan
    Status: captivity
    2 to 3 years


Mostly nocturnal or active at dusk, Norway rats go about digging burrows, foraging for food, and preparing nests during these hours. Often, these rats take up residence in areas near water. They are excellent swimmers, and are often referred to as "water rats." (Barnett, 1963; Calhoun, 1962; Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990)

Foraging behaviors can take the rats on long nightly excursions to areas known to be rich in food resources via learned routes. There is much learning capacity in Norway rats. They are able to remember their way around complex sewer and burrow networks. Their ability to learn has been thoroughly studied by psychologists. (Barnett, 1963; Calhoun, 1962; Parker, 1990)

New packs are started when a couple establishes a nest in a previously unoccupied area. Typically, R. norvegicus live in large, male-dominated groups. The heirarchy of such groups is based on the size of an individual. Nests are constructed of any efficient foraged materials including leaves, garbage, twigs, etc. Burrows are usually complex; consisting of food storage, nesting and "last ditch" chambers. (Barnett, 1963; Calhoun, 1962; Hamilton, 1998; Parker, 1990)

  • Average territory size
    2000 m^2

Home Range

Territories can be 50 meters in diameter. (Nowak and Paradiso, 1983; Parker, 1990)

Communication and Perception

Like most mammals, Norway rats use a variety of communication avenues. They are vocal, and also use visual cues such as body postures when communicating. Norway rats have relatively good hearing and tactile capabilities. They are able to sense very minute vibrations in the ground, and feel their way through total darkness with their paws and whiskers. However, these rats would not have such an advantage over all other foragers if it were not for their spectacular sense of smell. Scent is the Norway rats' best sensory channel, and it is used to find food and distinguish between individuals of a group. (Parker, 1990)

Food Habits

Norway rats are excellent foragers. Using their sense of smell and touch, they are able to survive quite easily given that there is a steady supply of any type of food. In metropolitan areas, they survive mainly on discarded human food, and anything else that can be eaten without negative consequences. Some Norway rats living near the sea have been observed catching fish with their paws. Also preyed upon by Norway rats are chicks, mice, birds, and small lizards. They have even been known to attack infant human beings.

Examination of a wild R. norvegicus stomach in Germany revealed 4000 items, most of which were plants, although studies have shown that Norway rats prefer meat when given the option. (Nowak and Paradiso, 1983; Parker, 1990; Nowak and Paradiso, 1983; Parker, 1990)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • fish
  • eggs
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • terrestrial worms
  • aquatic crustaceans
  • echinoderms
  • other marine invertebrates
  • zooplankton
  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems
  • seeds, grains, and nuts
  • fruit
  • nectar
  • flowers
  • sap or other plant fluids


Rattus norvegicus is preyed on by any number of carnivorous mammals, birds, and reptiles. Humans also kill very large numbers of Norway rats as pests. (Hamilton, 1998; Nowak and Paradiso, 1983; Silver, 1927)

Ecosystem Roles

Norway rats are excellent competitors and will readily drive out competing rat species, such as Rattus rattus. Because of their foraging habits, Norway rats act as seed dispersers. Their burrows also tend to aerate the soil. As prey, they help to sustain predator populations. Norway rats are commensal species with humans. (Hamilton, 1998; Nowak and Paradiso, 1983; Parker, 1990; Silver, 1927)

Mutualist Species

Economic Importance for Humans: Positive

Norway rats have been widely used in medical and genetic research. This research has led to important advances in physiology, genetics, immunology, pathology, and epidemiology. They are also popular pets and have been important in research on behavior because of their ability to learn quickly and because it is easy to keep them in laboratory settings. (Barnett, 1963; Calhoun, 1962)

  • Positive Impacts
  • pet trade
  • research and education

Economic Importance for Humans: Negative

Some consider Norway rats to be the greatest mammal pest of all time. They have caused more deaths than all the wars in history. Rat-borne diseases are thought to have killed more people in the last 1000 years than all of the wars and revolutions ever fought. They harbor lice and fleas that carry bubonic plague, typhus, trichinosus, tularemia, infectious jaundice, and many other serious diseases. These rats also cause considerable damage to property including crops, destroying and pollution of human food storage, and damage to insides and outsides of buildings. It is estimated that rats cause almost 1 billion dollars in damage in the United States each year. Rats kill poultry, domestic livestock, and game birds and are responsible for the endangerment or extinction of many species of wildlife, especially those found on islands. (Nowak and Paradiso, 1983; Silver, 1927)

Conservation Status

These animals are not a conservation concern. In fact, humans spend a great deal of effort trying to eradicate them.


Nancy Shefferly (editor), Animal Diversity Web.

Phil Myers (editor, instructor), Museum of Zoology, University of Michigan-Ann Arbor.

David Armitage (author), University of Florida, Wildlife Ecology and Conservation, 307 Newins-Ziegler Hall, PO Box 110430, Gainesville, FL 32611-0430.



Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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uses sound to communicate


living in landscapes dominated by human agriculture.


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.


flesh of dead animals.

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal


Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.


uses smells or other chemicals to communicate

cooperative breeder

helpers provide assistance in raising young that are not their own


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.


particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females


union of egg and spermatozoan


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


Referring to a burrowing life-style or behavior, specialized for digging or burrowing.


referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.


This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.


an animal that mainly eats all kinds of things, including plants and animals


found in the oriental region of the world. In other words, India and southeast Asia.

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pet trade

the business of buying and selling animals for people to keep in their homes as pets.


chemicals released into air or water that are detected by and responded to by other animals of the same species


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.


Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scent marks

communicates by producing scents from special gland(s) and placing them on a surface whether others can smell or taste them

scrub forest

scrub forests develop in areas that experience dry seasons.


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.

soil aeration

digs and breaks up soil so air and water can get in

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"


living in residential areas on the outskirts of large cities or towns.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


living in cities and large towns, landscapes dominated by human structures and activity.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Avalos, L., C. Callahan. 2001. "Classification and Characteristics of Mammals" (On-line). Accessed March 28, 2004 at http://www.humboldt.edu/~cmc43/mammalcharacters.htm.

Barnett, S. 1963. The Rat. Chicago & London: University of Chicago Press.

Calhoun, J. 1962. The Ecology and Sociology of the Norway Rat. Bethesda, MD: U.S. Department of Health, Education and Welfare.

Hamilton, W. 1998. The Mammals of Eastern United States, 3rd edition. Ithaca, NY: Comstock Publishing.

Nowak, R., J. Paradiso. 1983. Walker's Mammals of the World: Fourth Edition. Baltimore, MD: The Johns Hopkins University Press.

Parker, S. 1990. Grzimek's Encyclopedia of Mammals: Volume 3. New York: McGraw-Hill Publishing Company.

Silver, J. 1927. The Introduction and Spread of House Rats in the United States. Journal of Mammalogy, 8/1: 58-60.